A dielectric relaxation method for measuring the dielectric constant of a substance, an absorption spectrum measurement method for measuring the absorption distribution of an electromagnetic wave, and an ultrasonic propagating wave attenuation measurement method for measuring the amplitude damping factor of a passed ultrasonic wave can be used as a method for measuring gas concentration change and gas flow rate. Each of these methods does not have high time resolution.
On the other hand, a propagation time difference method for measuring the time of propagation of an acoustic wave from a transmitter to a receiver is a simple method, enabling the improvement of the time resolution. In this method, the transmitter transmits a standing wave such as a sine wave, the receiver receives the transmitted wave, the standing wave of the transmitter (transmitted wave) is compared with the signal from the receiver (received wave), a propagation time of the peak shift (phase difference) of these pulses is measured, and thus the propagation time corresponds to gas concentration change and gas flow rate.